The basic concept of the Stirling engine dates back to a patent registered by Robert Stirling in 1817. The engine operates by causing a working gas to shuttle between regions of temperature difference accompanied by volume and pressure variations. Stirling engines have a reversible thermodynamic cycle and therefore can be used as a means of delivering mechanical output power from a source of heat, or can act as a heat pump through the application of mechanical input energy. Using various heat sources such as combusted fossil fuels or concentrated solar energy, mechanical power can be delivered by the engine. This energy can be used to generate electricity or can be directly mechanically coupled to a load. Numerous potential applications for Stirling engines have been identified, for example including: as prime moves for motor vehicles, solar energy production, waste heat recovery, and remote location electricity generation.
The Assignee of the present application, STM Power, Inc. (previously named Stirling Thermal Motors, Inc.), has made significant advances in the technology of Stirling machines through a number of years. Examples of such innovations include development of a compact and efficient basic Stirling machine configuration employing a parallel cluster of double-acting cylinders which are coupled mechanically through a rotating swashplate. In many applications, a swashplate actuator is implemented to enable the swashplate angle and therefore the pistons' stroke and swept volume to be changed in accordance with engine operating requirements.
Although the Assignee has achieved significant advances in Stirling machine design, there is a constant need to provide further refinements. In a double-acting, multiple-cylinder Stirling engines, isolated volumes of the working fluid, typically helium or hydrogen gas, are shuttled through the engine. In accordance with the thermodynamic cycle of a Stirling engine, these isolate volumes are cyclically compressed and expanded and shuttled between spaces having temperature differences. Due to dynamic conditions during operation, leakage, and start-up conditions, changes in the mass of gas contained in each of the isolated cycle volumes occurs. These differences in “charge” mass or volume in the isolated cycle volumes lead to imbalances and roughness in operation of the machine. Moreover, such imbalances place undesired mechanical forces on the moving parts of the engine, increase noise and vibration of the engine during operation, negatively affects thermal efficiency, and increase starting torque.
Even with ideal sealing among the working parts of the Stirling engine and uniform charge volumes of the working gas during operation, once the engine is shut down, the cycle volumes will be stopped at various stages of compression. Inevitably, the working gas will leak from high pressure areas to low pressure areas over a period of time. This results in a difference in charge volume between cycles since each defines a separated volume. Thus, upon starting the engine, a significant difference in charge volume exists between working gas cycles. This invention provides a system for equalizing working gas charge volumes between the isolated cycle volumes.
One approach toward providing pressure balancing between isolated cycle volumes in a multiple-cylinder Stirling engine is described in Assignee's U.S. Pat. No. 5,813,229. That patent describes allowing each of the cycle volumes to communicate via a small diameter capillary tube. Although this system will result in pressure balancing over time, it has the disadvantage of creating a constant loss in efficiency due to an exchange of gas between cycles, even where pressure balance conditions do not exist. This occurs since the capillary tube is exposed to out-of-phase pressure variations and consequently there is a constant shuttling flow of gases through the capillary tubes.